test_firmware: Install all scripts

List all the scripts invoked by fw_run_tests.sh, so that
"make TARGETS=firmware install" keeps working.

Fixes: 29a1c00c ("test_firmware: add simple firmware firmware test ...")
Fixes: b3cf21fa ("test_firmware: test three firmware kernel configs ...")
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>